Magnetic Structure and Excitations in CeCu_xAl_4-x System

CeCuAl₃ crystallizing in the tetragonal BaNiSn₃-type structure and CeCu_xAl_4-x solid solutions were investigated by means of elastic and inelastic neutron scattering. Powder neutron diffraction brought information on both temperature evolution of crystallographic parameters and magnetic order at low temperatures. No structural change was observed in the investigated temperature range from 1.5 to 300 K. Weak magnetic peaks outside nuclear Bragg positions observed in solid solutions with 0.90 ≤ x ≤ 1.10 were described by the propagation vector k = (0.40 + δ_x, 0.60 + δ_y, 0), where δ_x ≈ 0.02 and δ_y ≈ 0.01. The magnetic structure of CeCu_0.75Al_3.25 consists of two components: an anti-ferromagnetic one described by the same k and a ferromagnetic one with k₀ = (0, 0, 0) and magnetic moments lying within the tetragonal basal plane. The evolution of magnetic excitations as a function of Cu-Al concentration in CeCu_xAl_4-x was studied by inelastic neutron scattering. The measured spectra of CeCuAl₃ and the solution with x = 0.95 point to a three-magnetic-peak energy scheme, while only two excitations are expected from the local symmetry conditions on Ce atoms. The standard two-peak spectrum of crystal electric field excitations was observed for Cu-Al substitutions further from the 1:1:3 stoichiometry (x = 0.75 and 1.10). The intermediate concentrations (x = 0.90 and 1.05) exhibit spectra on the border between the former cases with a less clear pronounced first inelastic magnetic peak. The observed behavior is discussed considering the evolution of structural parameters in the CeCu_xAl_4-x system and the coupling between the lattice vibrations and the crystal electric field excitations.